From 3f6874cb295695a225fca6825bfe24a4b97641fd Mon Sep 17 00:00:00 2001
From: Mitch Hayenga <mitch.hayenga@arm.com>
Date: Tue, 5 Apr 2016 11:44:27 -0500
Subject: cpu: Fix BTB threading oversight

The extant BTB code doesn't hash on the thread id but does check the
thread id for 'btb hits'.  This results in 1-thread of a multi-threaded
workload taking a BTB entry, and all other threads missing for the same branch
missing.
---
 src/cpu/pred/btb.cc | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

(limited to 'src/cpu/pred/btb.cc')

diff --git a/src/cpu/pred/btb.cc b/src/cpu/pred/btb.cc
index 393e52ccf..c7ef1959f 100644
--- a/src/cpu/pred/btb.cc
+++ b/src/cpu/pred/btb.cc
@@ -35,10 +35,12 @@
 
 DefaultBTB::DefaultBTB(unsigned _numEntries,
                        unsigned _tagBits,
-                       unsigned _instShiftAmt)
+                       unsigned _instShiftAmt,
+                       unsigned _num_threads)
     : numEntries(_numEntries),
       tagBits(_tagBits),
-      instShiftAmt(_instShiftAmt)
+      instShiftAmt(_instShiftAmt),
+      log2NumThreads(floorLog2(_num_threads))
 {
     DPRINTF(Fetch, "BTB: Creating BTB object.\n");
 
@@ -69,10 +71,12 @@ DefaultBTB::reset()
 
 inline
 unsigned
-DefaultBTB::getIndex(Addr instPC)
+DefaultBTB::getIndex(Addr instPC, ThreadID tid)
 {
     // Need to shift PC over by the word offset.
-    return (instPC >> instShiftAmt) & idxMask;
+    return ((instPC >> instShiftAmt)
+            ^ (tid << (tagShiftAmt - instShiftAmt - log2NumThreads)))
+            & idxMask;
 }
 
 inline
@@ -85,7 +89,7 @@ DefaultBTB::getTag(Addr instPC)
 bool
 DefaultBTB::valid(Addr instPC, ThreadID tid)
 {
-    unsigned btb_idx = getIndex(instPC);
+    unsigned btb_idx = getIndex(instPC, tid);
 
     Addr inst_tag = getTag(instPC);
 
@@ -106,7 +110,7 @@ DefaultBTB::valid(Addr instPC, ThreadID tid)
 TheISA::PCState
 DefaultBTB::lookup(Addr instPC, ThreadID tid)
 {
-    unsigned btb_idx = getIndex(instPC);
+    unsigned btb_idx = getIndex(instPC, tid);
 
     Addr inst_tag = getTag(instPC);
 
@@ -124,7 +128,7 @@ DefaultBTB::lookup(Addr instPC, ThreadID tid)
 void
 DefaultBTB::update(Addr instPC, const TheISA::PCState &target, ThreadID tid)
 {
-    unsigned btb_idx = getIndex(instPC);
+    unsigned btb_idx = getIndex(instPC, tid);
 
     assert(btb_idx < numEntries);
 
-- 
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